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In connexion with the elaboration of the chain of auditory
ossicles it is very usual for
mammals to possess a thin
inflated bone, sometimes
partly or entirely formed
out of the tympanic bone,
and known as the tympanic
bulla. Whether this struc-
ture is thin and inflated
or thick and depressed in
form it is characteristic of
the mammals, and does not
occur below them in the
series. But it is not pre-
sent in all mammals. It
is absent, for example, in
the Monotremes. When
it is present it is some-
times formed from other
bones, as, for instance, from
the alisphenoids. The tym-
panic ring has been held to
be the equivalent of the
quadrate. It is more prob- FIQ 16._Under S1,rface of t?ie cranium of a Dog.

a/pf, Anterior palatine foramen ; as, pos-

ably the quadrato-jugal.1

Ribs.  All mammals
are furnished with ribs, of
which, the number of pairs
differs considerably from
group to group, or it may
be even from species to
species. The ribs are
attached as a rule by two
heads, of which one, the
capitulum, arises as a rule
between two centra of
successive vertebrae. The

terior opening of alisphenoid canal; AS,
alisphenoid ; BO, basioccipital ; j8>S, basi-
spienoid; cf, condylar foramen,; earn, ex-
ternal auditory meatus ; JSss.O, exoccipital ;
Jim, foramen "lacerurn. medium ; jft/pt foramen
lacerum postering; fin>, foramen magnum;
fa, foramen ovale ; fr, foramen rotundum ;
JFV, frontal ; gf, glenoid fossa ; g$>, post-
glenoid process ; Ma, malar ; Mscy maxilla ;
oc, occipital condyle ; opy optic foramen ;
Per, mastoid portion of periotic ; pgf, post-
glenoid fossa ; JPl, palatine ; PJtfa:, pre-
maxilla ; ftp, paroccipital process ; ppf, pos-
terior palatine foramen ; PSt presphcnoid ;
Pt, pterygoid ; #f, Kphenoidal fissure or fora-
men lacerum aiiterius ; sm, stylomastoid fora-
men ; /SO, supraoccipitai j Sg, zygornatic
process of squamosal ; Tyt tympanic bulla;
Fa, vomer. (From Flower's Osteology.)

other,      the      tuberculum,
springs  from the transverse process.     Only in the Monotremes
1 Gegenbaur, Vergl. Anat.  Wvrbetth. Leipzig, 1898, p. 404.